To Build a SmartWatch… (Part 1)

Having seen the fuss around smart watche, I wondered exactly how difficult would it be to build one from scratch? All the technology usually packed in to one of the current generation of watches is widely available in breakout board form or equivalent, which brought me to another thought:

“How far can I get without building my own board?”

Thus, the Smart Watch project was born; an attempt to build a functional watch connected to a handset (in my case, a Nexus 5) and used to notify the user of incoming messages, etc. using only readily available hobby breakout boards and parts, so that as many people as possible could have a go at beating my designs!

If you, the reader, want to have a go, the rules are thus; The watch must:

  1. Only use readily available breakout boards to build it’s circuitry.
  2. Have the basic functionality of a smart watch, namely, to tell time, and show notifications from a host device.

Oh, and do let me know if you do have a go. I’d love to see the results!

The Parts

Because of the breakout board rule, I decided to base all my buying decisions around the single largest part required. As the display is likely to take up the lions’ share of the front surface of any watch, I attempted to cram all my other components behind that breakout.

The one I settled on in the end was the Adafruit 1.8″ TFT + SD Card Breakout with it’s whopping 128×160 resolution, 5v or 3.3v operation, and integrated Micro-SD card holder, it was a pretty easy choice to make – it gave me the added feature of storage, as well as providing a nice sized base to build the rest of the watch behind.

Conveniently, the stock image on Adafruit is a cat. I approve greatly.

The stock image on Adafruit’s display products is a slightly bemused looking cat. I approve greatly.

Conveniently, the processor board I had been looking to use for this project happened to be exactly the same height as the short dimension on the screen.

The Teensy 3.1 sports an ARM Cortex-M4 processor ( an MK20DX256VLH7 ), has hardware driven SPI, and has integrated USB support and handily runs a port of Arduino code, letting me use the Adafruit display libraries directly, rather than having to write my own ( although this came back to bite me later, but more on that another time ).

A Teensy 3.1! Nifty!

A Teensy 3.1! Nifty!

As this is to be a watch, it’s going to need to be battery powered, ( semi-obviously, I wasn’t keen on running about with a USB cable up my sleeve all the time ), and as small size was the main requirement when I decided on these parts, I tried to pick a battery with the largest capacity for approximately the right height available.

The 400mah LiPo battery from Sparkfun is about the right size, and should provide plenty of power for a working day ( a personal requirement of mine was that this should run for at least 8 hours )

It's a LiPo, what did you expect it would look like?

It’s a LiPo, what did you expect it would look like?

As the battery is a LiPo, it required it’s own charging circuitry, and luckily, the guy over at Pesky Products produces an ‘appallingly small’ ( their words, not mine! ) add-on board for the Teensy, which includes all the required circuitry, as well as breaking out a charge status pin and using the Teensy’s own USB connector for input power. It was perfect.

It's really tiny! Really handy too!

It’s really tiny! Really handy too! JST for scale?

Now that the device has power, a processor and something to display data on, a radio was needed to bridge to the host device. Adafruit once again provide the solution with their Bluetooth Low Energy, nrf8001 breakout which turned out to be the second largest single component to cram behind the screen ( the battery was the largest ) and proved to be quite a pain to mount sensibly.

Eventually I opted to put the board back-to-back with the display breakout, as thankfully, the radio breakout has no components on the back side, so could be simply stuck down with double sided foam ( more on the fun with sticky foam later… ). But even so, the sheer size of the breakout was – and still somewhat is – a problem.

The hilariously large - for my purposes - Adafruit nrf8001 breakout board.

The hilariously large – for my purposes – Adafruit nrf8001 breakout board.

More to come, when I can find the time to type everything up!

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